High-speed transmission technologies are a main theme in next-generation networking and computing systems. Their cost is an important problem to be solved to make the systems competitive in the market. A multi-layer substrate is one of the important techniques, which can satisfy both high-speed and cost-effective conditions within a predetermined frequency band and interconnection length. Electrical wiring in a planar conductor layer of the multi-layer substrate (board) can be developed on the basis of transmission lines such as microstrip lines, strip lines, and coplanar lines. Vertical interconnections in the board which serve mainly to connect planar transmission line structures disposed in different conductor layers are usually based on various types of via structures such as through-hole vias, blind vias, counter-bored, and buried vias.
Keeping the characteristic impedance within a predetermined level (within 10% as an example) through the whole path of an interconnection circuit between terminals is an important problem, especially in the design of the multi-layer substrate used in high-speed data transmission. This problem is equally related to both single-ended and differential signaling. Also, reduction of transformation from a differential mode to a common mode and from the common mode to the differential mode is another issue which has to be overcome in the high-speed multi-layer substrate. To improve the characteristic impedance control and to decrease leakage loss in a signal via structure in a higher frequency range, ground vias can be used around the signal via structure (single-ended or differential structure).
Also, electrical performance of the signal via structure is dependent on the shape and dimension of a clearance hole formed in an area for the signal via structure in conductor layers of the multi-layer substrate to provide passing a signal via without any electrical contact with other conductive surfaces in these conductor layers. As result, a transition from a via structure (single-ended or differential structure) to a planar transmission line becomes at the front because in this transition characteristic impedance mismatching can appear due to a large clearance hole. This impedance mismatching can excite a large reflection loss, a stray resonance and other unwanted effects. Moreover, in differential interconnection circuits, impedance mismatching can give a considerable increase of transformation between differential and common modes.
In Japanese Patent Application Publication (JP-P2004-363975A), a transition from a signal via pad to a coplanar transmission line interconnection is presented. In this related art, the transition has a form of coplanar transmission line having the width of a signal strip and distance to the ground plane providing its characteristic impedance matching to the coplanar transmission line interconnection. In Japanese Patent (JP-6-302964), a transition in the form of a linear taper connecting via pads (lands) of different diameters in a multi-layer board is shown. In Japanese Patent Application Publication (JP-P2003-218481A), a linear taper connecting planar transmission lines of different widths is presented. In US Patent Application Publication (US 2006/0091545A1), a transition from a differential signal via pair to a couple of transmission lines is provided by controlling the length of transmission line segments in the area of the clearance hole and also by an appropriate selection of the width of these segments.